The need for pointing devices goes back to the wartime days when radar operators and others needed ways of aiming guns. Given the connection between war and its highest tech expression, the aeroplane it isn't surprising that the first electronic pointing device was the joystick, closely followed by the light-pen.

The mouse took some years to think up and it wasn't until 1968 that Douglas Engelbart demonstrated a prototype of a device he called "X-Y position indicator for a display system". It was a wooden box with two wheels that could be trundled around a desktop. The name mouse followed later, supposedly because of the way the wire coming out of the back looked like a tail. And to think, we were only inches away from it being called a "truck" or a "wooden".

Dough Engelbart – father of the mouse.

The first mouse.

It's all relative

So what makes the mouse such a good idea?

Well the first thing to say is that straight after the mouse was invented it didn't really have much of an impact. It was more or less ignored until Steve Jobs visited the Xerox Parc research centre and "acquired" the ideas he needed to build the Mac. Apple was the first to popularise the mouse and it took the PC ages to catch up. In fact in the early days of the PC mouse there were a number of different ways of implementing a mouse and different mouse drivers. It took the advent of Windows to make the mouse standard issue on the PC and even then it was an "optional" extra for a long time after.

The key to the success of the mouse is the simple fact that it is a relative pointing device. It works in conjunction with an onscreen pointer which shows the current position. The mouse is simply used to move the pointer proportionally to the distance the mouse is moved. This probably seems obvious to you but the pointing devices that the mouse superseded didn't work in this way at all – they were absolute devices. The position of a joystick was mapped directly to a position on a screen and the light-pen actually picked a location on the screen by pointing at it. Compare this to the mouse where you can pick it up and move it to a more convenient location without altering the current screen position.

Mouse balls

There have been a number of different designs for converting movement into a signal that can be transmitted to a computer but the most common and most successful is the ball mouse.

A ball mouse uses a small ball and a pair of rollers to convert the movement into a signal. As the ball is rolled over the surface it turns the horizontal and vertical roller, thus converting the motion into separate horizontal and vertical rotations.

Balls and rollers – that's all a mouse is

As the rollers go round they turn a small wheel with slots in it. This is used to generate the digital signal that tells the PC how far the mouse has moved by interrupting the light from a small LED. Each time a slot moves through the beam the result is a pulse.

Rotation to pulses

All this is simple enough but there is a mystery here. How does the PC know which direction the wheel is moving in? The answer is that there are in fact two LED sensor pairs and these are positioned such that they see the slots at slightly different times. As the wheel rotates the electronics in the mouse detects the direction by noticing which pair of LED sensors sees the light first.